I'm eagerly awaiting the 7.8 update (although realistically I expect to upgrade to a WP8 device before 7.8 is even available from my carrier), but my wife has expressed distaste for the larger tiles based on what she's seen in commercials. Since I'm her "IT pro" I always make sure here phone and laptop are up-to-date with the latest security and stability enhancements.

To my knowledge 7.8 brings the updated start screen and little else. Since she's not due to upgrade for another year, my question is would she be missing anything important (again, such as security or stability fixes) by not upgrading to 7.8?

I think you question might be better if it was worded "What new features and fixes does 7.8 bring?" As it stands 'how necessary' can't really deliver a concrete answer.
–
row1♦Feb 19 '13 at 8:45

I should have added she has an LG Quantum. She also likes the margin on the right of the start screen, so while she understands tiles can be resized, she doesn't like the layout. And by "necessary" I was stressing security and stability fixes. It doesn't appear 7.8 has many if at all.
–
Michael ItzoeFeb 19 '13 at 17:11

As well as major system updates, there has been a stream of exciting
new features introduced to Nokia Lumia. These include Internet
sharing, Bluetooth sharing, imaging enhancements like Camera Extras,
and the release of apps such as Contact Share, Play To and Nokia City
Lens that are available.

I've found the Bing translate live tile with the word of the day suffers from this issue, so it is a case of removing the tile until this issue has been resolved.
Otherwise, the ability to resize tiles is great, you can fit more on one screen.